This invention relates to AC calibration apparatus.
Various methods are in use world wide for the accurate measurement of AC mains voltage or current, and attendant quality such for example as power metering and power quality measurement. One known method involves taking multiple samples of an input wave (voltage or current) during a mains cycle, converting each sample to digital form with an analogue to digital converter, and performing mathematical operations on the results to accurately assess root mean square voltage (or current) or any other parameter which may be desirable. In one class of known system, the samples are taken regularly at a rate Fs consistent with sampling theory. The resultant digital values are transformed from a time domain to a frequency domain to yield more easily the desired parameters. In this known class of system and in other systems, the accuracy of the analogue to digital converter is crucial to the accuracy of the final result. One problem is that the absolute gain accuracy of the analogue to digital converter may vary with factors such for example as temperature or age. The dynamic variation of the analogue to digital converter gain is usually assessed in known apparatus by occasionally presenting a DC signal of known amplitude to the analogue to digital converter in place of the normal input signals, and using the corresponding converted digital value as a reference against which the AC measurements can be scaled.
The existing methods as described above cause problems in that system characteristics commonly affecting the desired AC signal but not the reference DC signal cannot be dynamically removed by the above described reference assessment process. AC calibration systems are also used but these are usually a final part of the production process employing external generators. Such AC calibration systems are either not economic or, if built-in, are not accurate enough to be useful for dynamic testing.
It is an aim of the present invention to obviate or reduce the above mentioned problems.
Accordingly, in one non-limiting embodiment of the present invention there is provided AC calibration apparatus comprising an analogue to digital converter, and in which the absolute gain of the analogue to digital converter is dynamically assessed by applying a reference AC signal which is in addition to a sampling signal required for measurement purposes and which is a square wave of a known amplitude at a precise sub-harmonic of the sampling frequency of the analogue to digital converter.
Thus, the present invention is able accurately to assess the gain of the analogue to digital converter in AC calibration apparatus by applying an AC signal of special characteristics at the same time as normal input sampling signals, and such that an accurate reference measurement is able to be made which is more representative of normal operation.
The AC calibration apparatus will normally be one in which the reference AC signal is applied at the same time as the sampling signal.
Preferably, the AC calibration apparatus is one in which the edge of the square wave is alternately aligned with the sampling signal such that the reference AC signal has a frequency of Fs/2.
Also preferably, the analogue to digital converter is a Sigma-Delta analogue to digital converter. Such Sigma-Delta analogue to digital converters have more difficulty with DC than AC, although some such Sigma-Delta to analogue digital converters do achieve good DC performance. However, they often require off-line offset measurement cycles to effectively calibrate out the offset imperfections. In the case of one known inexpensive Sigma-Delta analogue to digital converter, the inherent DC performance is immaterial because the manufacturer has chosen to digitally remove the long term DC part of any measurement. Thus, with such a converter in the AC calibration apparatus of the present invention, DC is actually rejected. This means that the input signal can be continuously measured, which is important in the present invention. If desired, a Sigma-Delta analogue to digital converter may be used which does pass DC, provided its off-line offset calibration process can either be suppressed, or does not deny time to the proper input signal measurement. The considerable advantage of the present invention is the independence of both DC offset and gain variation.
Generally, the AC calibration apparatus may be one in which the analogue to digital converter does not pass DC, and in which the analogue to digital converter also has an inherent input anti-alias digital filtering means which scales with sampling frequency. Because the analogue to digital converter does not pass DC, a DC reference signal cannot be employed. Since the inherent input anti-alias digital filtering means scales with sampling frequency, any attenuation of any related frequency is exactly known. Further, with certain analogue to digital converters, the attenuation which makes such analogue to digital converters easily usable at the preferred reference AC signal frequency of Fs/2. If such a frequency cannot conveniently be used, then the reference AC frequency may be Fs/4, but such a frequency may introduce more problems than the reference AC signal frequency Fs/2. The Sigma-Delta analogue to digital converters are especially preferred since they are relatively inexpensive to purchase.
The AC calibration apparatus may be one in which an input amplifier combines the sampling signal with the reference AC signal and then inputs the combined signals to the analogue to digital converter.
The AC calibration apparatus may be one in which the analogue to digital converter outputs to a processor, in which the processor outputs to a sampling frequency control system, and in which the sampling frequency control system outputs to the input amplifier via switched reference generation means. The switched reference generation means may include first and second switches arranged such that one switch is open when the other switch is closed and vice versa, in order to give a break before make operation.
An embodiment of the invention will now be described solely by a way of example and with reference to the accompanying drawings in which.